It is well known that organic polyhydroxy compounds having hydroxyl groups positioned in the cis-form on adjacent carbon atoms or on carbon atoms in a 1,3-relationship react with borates to form five or six member ring complexes. At alkaline pH above about 8.0 these complexes form didiol crosslinked complexes, i.e., ##STR1##
This leads to a valuable reaction with dissociated borate ions in the presence of polymers having the required hydroxyl groups in a cis-relationship. The reaction is fully reversible with changes in pH. An aqueous solution of the polymer will gel in the presence of borate when the solution is made alkaline, and will liquify again when the pH is lowered below about 8. If the dry powdered polymer is added to an alkaline borate solution, it will not hydrate and thicken until the pH is dropped below about 8. The critical pH at which gelation occurs is modified by the concentration of dissolved salts. The effect of the dissolved salts is to change the pH at which a sufficient quantity of dissociated borate ions exists in solution to cause gelation. The addition of an alkali metal base such as sodium hydroxide enhances the effect of condensed borates such as borax by converting the borax to the dissociated metaborate.
Known polymers which contain an appreciable content of cis-hydroxyl groups are exemplified by guar gum, locust bean gum, dextrin, polyvinyl alcohol, and derivatives of these polymers. Derivatives tend to react less with borate ions as the amount of substituting groups in the molecule increases. This results because the shear bulk of substituting groups changes the regular, alternating, and single-member branched, linear configuration of the molecule and prevents adjacent chains from approaching as closely as before, and the substitution of secondary cis-hydroxyl positions decreases the number of such unsubstituted positions available for complexing with the borate ion.
Strong reactions of such polymers are also obtained with solutions of certain inorganic cations. The addition of a high concentration of calcium salt, for example, will cause a polymer gel to form under alkaline conditions. If dry powdered polymer is added to the salt solution, the polymer will not generally hydrate and thicken. In general, the polymer will react with polyvalent cations much as it does with borate anions.
Depending on the relative concentration of polymer, and borate anion or polyvalent cation, the crosslinking reaction may produce useful gels, or may lead to insolubilization, precipitation, or unstable, non-useful gels. The viscosity of the hydrated polymer solution increases with an increase in the concentration of borate anion until a maximum is obtained. Thereafter the viscosity decreases and the gel becomes unstable as evidenced by a lumpy, inhomogeneous appearance and syneresis. As the temperature of the solution increases, the concentration of borate required to maintain the maximum degree of crosslinking, and thus maximum viscosity increases. Derivatization with non-ionic hydroxyalkyl groups greatly improves the compatibility of the polymer with most salts.
Hydraulic fracturing is a widely used method for stimulating petroleum producing subterranean formations and is commonly performed by contacting the formation with a viscous fracturing fluid having particulated solids, widely known as propping agents, suspended therein, applying sufficient pressure to the fracturing fluid to open a fracture in the subterranean formation, and maintaining this pressure while injecting the fracturing fluid into the fracture at a sufficient rate to extend the fracture into the formation. When the pressure is reduced, the propping agent within the fracture prevents the complete closure of the fracture.
The properties that a fracturing fluid should possess, are amongst others, low leakoff rate, the ability to carry a propping agent, low pumping friction loss, and it should be easy to remove from the formation. Low leakoff rate is the property that permits the fluid to physically open the fracture and one that controls its areal extent. The rate of leakoff to the formation is dependent upon the viscosity and the wall-building properties of the fluid. Viscosity and wall-building properties are controlled by the addition of appropriate additives to the fracturing fluid. The ability of the fluid to suspend the propping agent is controlled by additives. Essentially, this property of the fluid is dependent upon the viscosity and density of the fluid and upon its velocity. Friction reducing additives are added to fracturing fluids to reduce pumping loss due to friction by suppression of turbulence in the fluid. To achieve the maximum benefits from fracturing, the fracturing fluid must be removed from the formation. This is particularly true with very viscous fracturing fluids. Most of such viscous fluids have built-in breaker systems that reduce the viscous gels to low viscosity solutions upon exposure to the temperatures and pressures existing in the formations. When the viscosity is lowered, the fracturing fluid may be readily produced from the formation.
The use of aqueous based fluids to formulate fracturing fluids is well known. Such fluids generally contain a water soluble polymer viscosifier. Sufficient polymer is used to suspend the propping agent, decrease the leakoff rate, and decrease the friction loss of the fracturing fluid. Supplemental additives are generally required to further decrease the leakoff rate, such as hydrocarbons or inert solids, such as silica flour.
Various water soluble polymers have been proposed for use as viscosifiers for aqueous based fracturing fluids, such as polyacrylamides, partially hydrolized polyacrylamides, and various polysaccharide polymers such as guar gum and derivatives thereof, and cellulose derivatives. However, guar gum and guar gum derivatives are the most widely used viscosifiers. Guar gum is suitable for thickening both fresh and salt water, including saturated sodium chloride brines. At least two basic types of guar gum formulations are used to obtain a desirable gelled water-base fluid. These are classified as materials suitable for batch mix operations and materials suitable for continuous mix operations. The most widely used form is the continuous mix grade which hydrates rapidly and reaches a useable viscosity level fast enough that it can be added continuously as the fluid is pumped down the well. This grade of guar gum has a very small particle size. The easy mixing or batch mix grades of guar gum are designed to take advantage of the complexing action of guar gum with borax. In the presence of borax, the guar gum can be dissolved in a slightly alkaline solution without increasing the viscosity of the solution. Thus these easy mixing grades of guar are alkaline mixtures of guar gum and borax with a delayed-action acid.
There is disclosed in U.S. Pat. No. 3,974,077 a method of ultilizing the crosslinking reaction of borates with guar gum in a continuous mix process. The method comprises adding a compound for releasing borate ions into a flowing stream of aqueous fluid prior to or substantially concurrently with the addition to the fluid of a galacto-mannan gum, a buffer which produces an initial acidic pH in the fluid, and a delayed action basic compound to neutralize the buffer and produce a basic pH in the fluid after a period of time sufficient to permit introduction of the fluid into the well while still acidic and of low viscosity, and yet within a time period such that the guar gum hydrates and is crosslinked prior to reaching the formation. Thus the gelation time, or crosslinking time, is dependent upon the solubility rate of the delayed action basic compound and the time required to neutralize the acidic buffer.
Thus the crosslinking reaction of borate ions with guar gum or similar polymers having hydroxyl groups in a cis-relationship has been used in preparing fracturing fluids in the following manner:
(1) adding the polymer to an alkaline borate solution to prevent the polymer from hydrating, with subsequent lowering of the pH to form an acidic solution, thus breaking the borate crosslinking and allowing the polymer to hydrate; PA0 (2) adding the polymer to a borate containing solution with an acidic buffer and a delayed action (slowly soluble) basic compound such that the polymer hydrates at the acidic pH before the pH is raised initiating the crosslinking reaction.
I have disclosed in copending International Patent Application No. PCT/US83/01408 the use of hydrated borates as bridging agents in well drilling, workover and completion fluids, including fracturing fluids.